This research proposal utilizes Plasmodium yoelii, a urine malarial parasite which requires antibodies for resolution of infection. A monoclonal antibody (McAb 302) was constructed which provided dramatic passive protection of mice infected with the erthrocytic stage of this parasite. This antibody possessed the unusual IgG3 isotype and recognized the major merozoite surface antigen, PY230. Subsequent studies have been concerned with the idiotypy of McAb 302, the mechanism responsible for its biological activity and the plasmodial antigen which it recognizes. A portion of the gene encoding the epitope recognized by McAb 302 has been molecularly cloned and sequenced. This analysis has established that the epitope is within the C-terminal portion of the PY230 antigen and that this area of the molecule has significant homology with the PF195 antigen of P. falciparum. Such data supports the contention that the C-terminal region, which is more conserved between strains, contains epitopes which may be important in the induction of protective immune responses to the major merozoite surface antigen. This hypothesis can be approached using the murine model. Several techniques will be utilized to map the epitope recognized by McAb 302. In addition, the remainder of the gene encoding PY230 will be cloned and sequenced. Since the PF195 antigen of P. falciparum is known to be polymorphic, the major merozoite protein will be characterized in other murine plasmodia. These studies will provide the basis for determining important T-cell and B-cell epitopes of PY230. Preliminary studies with bacterial expression plasmids have shown that recombinant PY230 polypeptides can be synthesized in bacterial hosts and subsequently purified. Defined recombinant or synthetic peptides will be used in immunization studies in mice examining alternate conditions of administration and different host strains. Animals determined to be immune to parasite challenge will be further studied to establish whether the protective immunity induced is a reflection of cell-mediated or humoral mechanisms of immunity. This system thus provides a unique model or examining the interaction of the host immune system with defined epitopes of one of the most important malarial antigens described to date.